When the New Model Army formed in the
beginning of 1645, Colonel Lambert was appointed to succeed
Sir Thomas
Fairfax in command of the northern forces, with the title of
commissary-general. Fairfax was soon replaced by Sydnam Poyntz, and under this officer Lambert
served in the Yorkshire campaign of 1645, receiving a wound before
Pontefract.In 1646 he was given a regiment in the New
Model, serving with Sir Thomas in the west of England, and he was a
commissioner, with Cromwell and others, for the surrender of
Oxford in the same year. "It is evident", says
Charles Harding Firth (in the
Dictionary of
National Biography), "that he was from the first regarded
as an officer of exceptional capacity and specially selected for
semi-political employments".

When the quarrel between the Army and Parliament began, Lambert
supported the Army's cause. He assisted Henry Ireton in drawing up the addresses and
remonstrances issued by the Army, both men having had some
experience in law. Early in August 1647 Lambert was sent by Fairfax
as Major-General to take
charge of the forces in the northern counties. His management of
affairs in those parts is praised by Whitelocke. He suppressed a mutiny
among his troops, kept strict discipline and hunted down the
moss-troopers who infested the moorland country.

At the start of the Second
English Civil War Lambert now a young general of twenty-nine,
was more than equal to the situation. He had already left
the sieges of Pontefract
Castle and Scarborough Castle to Colonel Edward Rossiter, and hurried into
Cumberland to deal with the English Royalists under Sir
Marmaduke Langdale.With his
cavalry he got into touch with the enemy about Carlisle and slowly fell back, fighting small rearguard
actions to annoy the enemy and gain time, to Bowes and Barnard Castle. Langdale did not follow him into the
mountains, but occupied himself in gathering recruits and supplies
of material and food for the Scots. Lambert, reinforced from the
midlands, reappeared early in June and drove him back to Carlisle
with his work half finished. About the same time the local horse of
Durham and Northumberland were put into the field by Sir Arthur Hesilrige, governor of Newcastle, and under the command of Colonel Robert Lilburne won a considerable success
(June 30) at the River
Coquet.

This reverse, coupled with the existence of Langdale's force on the
Cumberland side, practically compelled Hamilton to choose the west
coast route for his advance, and his army began slowly to move down
the long couloir between the mountains and
the sea. The campaign which followed is one of the most brilliant
in English history. When the Scottish army under the James Hamilton, 1st Duke of
Hamilton invaded England in the summer of 1648, Lambert was
obliged to retreat; but Lambert continued to harass the invaders
till Cromwell came up from Wales and the
Scottish army was destroyed in the three days' fighting at the
Battle of
Preston fought largely at Walton-le-Dale near Preston in Lancashire.After the battle Lambert's cavalry headed
the chase, pursuing the defeated army, and finally surrounded it at
Uttoxeter, where Hamilton surrendered to Lambert on August 25, 1648.
Lambert then led the advance of Cromwell's army into Scotland,
where he was left in charge on Cromwell's return. From December 1648 to
March 1649 he was engaged in the successful siege of Pontefract
Castle; Lambert was thus absent from London at the time of
Pride's Purge and the trial and
execution of King Charles
I.

When Cromwell was appointed to the command of the war in Scotland
(July 1650), Lambert went with him as major-general and second in
command. He was wounded at Musselburgh, but returned to the front in time to take a
conspicuous share in the victory of Dunbar.He himself repulsed a surprise attack by the
Covenanters at the battle of Hieton, Hamilton on 1 December1650.In July 1651 he was sent into Fife to get in
the rear and flank of the Scottish army near Falkirk, and force them to decisive action by cutting off
their supplies.This mission, in the course of which Lambert
won an important victory at Inverkeithing, was so successful that Charles II, as Lambert had foreseen,
made for England.Lambert's part in the general plan of the
resulting Worcester campaign was carried out brilliantly, and in
the crowning victory of Worcester he commanded the right wing of the English army,
and had his horse shot under him. Parliament granted him
lands in Scotland worth £1000 per annum.

Political career

In
October 1651 Lambert was made a commissioner to settle the affairs
of Scotland, and on the death of Ireton he was appointed lord
deputy of Ireland (January 1652). He made extensive
preparations; parliament, however, reconstituted the Irish
administration and Lambert refused to accept office on the new
terms. He then began to oppose the Rump
Parliament. In the Council of
Officers he headed the party desiring representative
government, as opposed to Harrison who favoured an oligarchy of "God-fearing" men, but both hated the
Rump of the Long Parliament, and
joined in urging Cromwell to dissolve it by force.

At the same time Lambert was consulted by the parliamentary leaders
as to the possibility of dismissing Cromwell from his command, and
on 15 March, 1653
Cromwell refused to see him, speaking of him contemptuously as
"bottomless Lambert". On 20 April, 1653, however, Lambert accompanied Cromwell when he
dismissed the Council of
State, on the same day as the forcible expulsion of the
parliament.

Lambert now favoured the formation of a small executive council, to
be followed by an elective parliament whose powers should be
limited by a written instrument of government. As the ruling spirit
in the Council of State, and the idol of the army, he was seen as a
possible rival of Cromwell for the chief executive power, while the
royalists for a short time had hopes of his support. He was
invited, with Cromwell, Harrison and John Desborough, to sit in the nominated
"Barebones Parliament" of 1653;
and when the unpopularity of that assembly increased, Cromwell drew
nearer to Lambert. In November 1653 Lambert presided over a meeting
of officers, when the question of constitutional settlement was
discussed, and a proposal made for the forcible expulsion of the
nominated parliament. On 12 December,
1653, the parliament resigned its powers into
Cromwell's hands, and on December 13 Lambert obtained the consent
of the officers to the Instrument of Government, in
the framing of which he had taken a lead. He was one of the seven
officers nominated to seats in the council created by the
Instrument.

In the
foreign policy of the Protectorate
Lambert called for alliance with Spain and war
with France in 1653,
and he firmly withstood Cromwell's design for an expedition to the
West
Indies. In the debates in parliament on the
Instrument of Government in 1654 Lambert proposed that the office
of Lord Protector should be made
hereditary, but was defeated by a majority which included members
of Cromwell's family. In the parliament of this year, and again in
1656, Lord Lambert, as he was now styled, sat as member for the
West Riding. He was one of
the major-generals appointed in August 1655 to command the militia in the ten districts into which it was
proposed to divide England, and who were to be responsible for the
maintenance of order and the administration of the law in their
several districts.

Lambert took a prominent part in the Committee of Council which drew up
instructions to the administrative major-generals.
He was the organiser of the system of police which these officers
were to control. Samuel
Gardiner conjectures that it was through divergence of opinion
between the protector and Lambert in connection with these
"instructions" that the estrangement between the two men began. At
all events, although Lambert had himself at an earlier date
requested Cromwell to take the royal dignity, when the proposal to
declare Oliver king was started in parliament (February 1657) he at
once opposed it.

A hundred officers headed by Charles
Fleetwood and Lambert waited on the protector, and begged him
to put a stop to the proceedings. Lambert was not convinced by
Cromwell's arguments, and their complete estrangement, personal as
well as political, followed. On his refusal to take the oath of
allegiance to the protector, Lambert was deprived of his
commissions, receiving instead a pension of £2000 a year.
He
retired from public life to Wimbledon; but shortly before his own death Cromwell sought a
reconciliation, and Lambert and his wife visited Cromwell at
Whitehall.

When Richard Cromwell was
proclaimed protector (3 September1658), his chief difficulty lay with the army,
over which he exercised no effective control. Lambert, though
holding no military commission, was the most popular of the old
Cromwellian generals with the rank and file of the army, and it was
very generally believed that he would install himself in Oliver
Cromwell's seat of power. Richard
Cromwell's adherents tried to conciliate him, and the royalist leaders made overtures to him, even
proposing that Charles II
should marry Lambert's daughter. Lambert at first gave a lukewarm
support to Richard Cromwell, and took no part in the intrigues of
the officers at Fleetwood's residence, Wallingford House. He was a
member of the Third
Protectorate Parliament which met in January 1659, and when it
was dissolved in April under compulsion of Fleetwood and
Desborough, he was restored to his commands. He headed the
deputation to Lenthall in May 1659
inviting the return of the Rump
Parliament, which led to the tame retirement of Richard
Cromwell; and he was appointed a member of the Committee of Safety and of the
Council of State.

When the parliament, in an attempt to control the power of the
army, withheld from Fleetwood the right of nominating officers,
Lambert was named one of a council of seven charged with this duty.
The
parliament's evident distrust of the soldiers caused much
discontent in the army; while the absence of authority encouraged
the royalists to make overt attempts to restore Charles II, the
most serious of which, under Sir George Booth and the
earl of Derby,
was crushed by Lambert near Chester on 19 August1659. He promoted a petition from his army that
Fleetwood might be made lord-general and himself major-general. The
republican party in the House took offence. The Commons (12 October1659) cashiered Lambert and other officers, and
retained Fleetwood as chief of a military council under the
authority of the speaker. On the
next day Lambert caused the doors of the House to be shut and the
members kept out. On 26 October a new
Committee of Safety was appointed, of which he was a
member. He was also appointed major-general of all the forces in
England and Scotland, Fleetwood being general.

Lambert was now sent with a large force to meet George Monck, who was in command of the English
forces in Scotland, and either negotiate with him or force him to
terms. Monck, however, marched southward. Lambert's army began to
melt away, and he was kept in suspense by Monck till his whole army
deserted and he returned to London almost alone. Monck marched to
London unopposed. The excluded Presbyterian members were recalled.
Lambert
was sent to the Tower (3 March1660), from which he
escaped a month later. He tried to rekindle the civil war in
favour of the Commonwealth
by issuing a proclamation calling on all supporters of the
"Good Old Cause" to rally on the
battlefield of Edgehill. But he was recaptured on 22 April at Daventry by Colonel Richard Ingoldsby, a regicide who hoped to win a pardon by handing
Lambert over to the new regime . He was kept imprisoned in the Tower of
London and then transferred to Castle Cornet on the island Guernsey.

On the Restoration Lambert was
exempted from prosecution by an address of both Houses of the
Convention Parliament to the
king, but the Cavalier
Parliament in 1662 charged him with high treason. In April 1662 General Lambert
was, with Sir Henry Vane,
brought to England and tried in June 1662. On 25 July a warrant was
issued to Lord Hatton,
the governor of
Guernsey, to take into his custody "the person of John Lambert,
commonly called Colonel Lambert, and keep him a close prisoner as a
condemned traitor until further orders." On 18 November following,
directions were given from the king to Lord Hatton to "give such
liberty and indulgence to Colonel John Lambert within the precincts
of the island as will consist with the security of his
person".

It has been said that Lambert's nature had more in common with the
royalist than with the puritan spirit. Vain and ambitious, he believed that
Cromwell could not stand without him; and when Cromwell was dead,
he imagined himself entitled to succeed him. As a soldier he was
far more than a fighting general and possessed many of the
qualities of a great general. He was an able writer and speaker, and an
accomplished negotiator and took pleasure in quiet and domestic
pursuits. He learnt his love of gardening
from Lord Fairfax, who was also his master in the art of war. He
painted flowers, besides cultivating them,
and was accused by Mrs Hutchinson of
"dressing his flowers in his garden and working at the needle with
his wife and his maids". On his death bed, some Catholic detractors
claim that he renounced Protestantism, however the Cromwell
association has disproved this myth.